My good friend, Gary Fitch, acquired this amp a few years ago. Although it was in working condition when it arrived, there were obvious signs that this little amp needed some love and attention. Not only were there physical signs that someone else had previously attempted to overhaul this amp (i.e. speaker replacement, flat-wire power cord, a few bad circuit modifications, etc…) the sound had some issues as well, including a noticeable buzz in the Normal channel and a lack of that classic punchy Fender clean sound. It also seemed to have a low-end flutter, often caused by bad filter caps.

Before we get into the overhaul work, let’s review the specs of this relatively unknown Fender guitar amplifier.

History

Pro Reverbs were first sold in mid-1965. CBS bought Fender in early 1965, which means that most production units were post-CBS era even though the early models included a “black” faceplate with script writing. Early production models possessed the "Fender Electric Instruments Co." on the front panel under the Pro Reverb Amp logo instead of the CBS-era "Fender Musical Instruments." All the original Pro Reverbs were "black face" (i.e. they have black control panels with white lettering, script logo, silver/black/white grill cloth, raised Fender logo on the grill cloth and black Tolex covering).

Technically, these amps used two 5881/6L6GC power tubes and were rated at roughly 40 Watts. They have two channels: one of which had both reverb and tremolo (not vibrator) and one that included a dual footswitch to control the effects.

In 1967 Fender changed the look of this amp to match its current style. The front panel was polished aluminum (silver face) and included blue lettering. The grill cloth was changed to a turquoise weave. The cabinet depth was increased by 1 inch in depth and became more angular around the front panel. An aluminum trim strip surrounded the speaker panel (a feature I personally find ugly) was also added, but only last about a year in production.

The early black face Pro Reverb amps used the AA165 circuit until early 1968 when the first silver face versions came out using a new AB668 circuit, which had changes to the bias section and changes to the capacitors off the grids of the power tubes, and resistors off the cathodes. These added “modifications” to the AB668 circuit were implemented to improve stability, improve sound, and lower the production cost. However, due to poor sound quality the cathode modification was changed again in 1970, and the AA1069 circuit was introduced and used for new production models.

In 1972 the Silverface Pro Reverb went from a 40-watt to a 45-watt amp. The stock rectifier changed from a 5U4GB to a GZ34. In 1975 Fender added a master volume control and a push-pull distortion pot as well as adding a midrange control to the Vibrato channel. At some point in the mid-70's the cabinet size was increased again so the Twin and Pro could share the same cabinet (again to reduce production costs).

As far as speakers were concerned, most Pro Reverbs came stock with Utah speakers. Gary’s amp is fitted with Celestion 30s, which I’m sure were a replacement for the originals at some point.

As a rule of thumb, I always start with a thorough play through of the amp. I note all things that could be wrong (dirty pots, ticking, buzzing, flabby tone, etc…). I play through all channels and turn every knob. As I begin work, I will always do one upgrade or modification at a time and then play through the amp to make sure it’s still working. If you perform too many modifications and the amp doesn’t work, it is very difficult to trace your steps backwards to find the problem. It’s a good rule of thumb, for me anyway, and, of course, I document each step along the way.

Mod 1 – Filter Caps

I almost always start with the filter section. If it’s an old amp with old filter caps, I just replace them all with brand new Sprague Atom caps. This gives me comfort that the power supply section will be operating properly and will be stable for the rest of my work.

Filter capacitors are part of the power supply and bias supply in a guitar amp and will dry out over time. Bad filter caps can cause the amp to have “ghost notes” and a flabby bottom end. As you can see from the pictures, the old caps are bursting on the end. Another problem that old filter capacitors can cause is leaking your plate voltage to ground. This will give you less power and punch.

Some players like these sounds caused by bad filter caps. I’ve read on a few forum sites that some players prefer bad caps in their amp and refuse to change them so they will not lose their “Mojo” tone. It’s silliness really. When there are bad filter caps, the amp will continue to blow its fuse. If you don’t address the problem, bigger issues will arise and the amp will blow eventually.

I typically use Sprague Atom filter capacitors. These are great caps at a low cost. However, Sprague does not make values that cover all amps. In this case, I did not upgrade the filter cap specification. The resistors were in tolerance so I left them in place.

Someone had previously tried to upgrade this bias circuit. It no longer followed the AA165 schematic. Although it worked fine, I decided to revert back to the original circuit design and stay true to the blackface (AA165) schematic.

I installed a proper 470ohm resistor (1w) in the circuit. I also replaced the bias capacitor, another standard modification I almost always do. A bad bias capacitor will cause your amp to create small audible oscillations and will sound like a strange tremolo or vibrato. What’s worse is if the bias filter cap shorts out you will burn up your power tubes!

Below is the work I completed:

•Replaced Bias Resistor from 27K to 470 at 1w

•Replaced Bias Capacitor (new 47uF)

•Replaced resistor on 10K pot to ground (new 27K)

•Replaced driver resistors (new 220K)

Mod 3 – Tone Circuits

As a matter of practice, I upgrade components in the tone circuit on both channels. Tone circuit capacitors typically drift out of tolerance and replacing them with good quality capacitors can make a world of difference. I use high quality Orange Drop capacitors for the bass and middle tone circuit, and I almost always replace the treble cap with a new Silver Mica capacitor. This gives me a nice sparkling high-end control.

Many amps use an electrolytic cap on the cathodes of preamp tubes, or cathode biased power tubes. The function of this cap is to help decrease sag and mush when the amp is pushed. Most Fenders use a cathode bypass cap when the tube is arranged in a standard plate follower configuration. Since these are electrolytic caps, they are subject to drying out. This can cause shorting, which will tend to burn up the particular tube that the cap is on. They can also start to drift upward in value that can cause a decrease in headroom and distortion (although, sometimes we want this distortion...)

One of the most important modifications for me is to upgrade the Cathode bypass capacitors on the preamp stages. Many old Fenders used the old blue-blob caps. This amp used a dual capacitor component (two 25uF caps in a single package). I replaced them all with new Sprague coaxial caps. In addition, the second stage of the preamp circuit (after the tone stack) shares the bypass capacitor circuit using an 820ohm resistor. I always separate this circuit and add an additional bypass circuit for the second channel. I use a 25uF capacitor in parallel with a 1.5Kohm resistor for each circuit. This makes the both channels perform a little better.

Coupling capacitors are probably the most important capacitor that affects your tone. Coupling capacitors transfers your guitar signal from one stage to the next. It’s a bottleneck experience so your entire tone goes through this cap.

Again, as standard practice, I like to replace these caps with the highest quality capacitor necessary to keep your tone alive and healthy from one stage to the next. Good quality Orange Drop caps work well for this upgrade.

Below is the work I completed:

•Replaced both coupling caps for both channels. Used high quality Orange Drop caps for the .022uF @ 400V (Vibrato Ch.) and the .047uF @ 400V (Normal Ch.).

Mod 6 – Phase Inverter

As with the bias circuit on this amp, someone previously tried to modify the inverter circuit. It no longer resembled the AA165 schematic. I decided to go back to the original design.

There are also important phase inverter modifications that are critical to not only making your amp run perfectly, but to perfect tone quality. For instance, the two (.1uF) capacitors on each end of the phase inverter circuit must be matched or as close as possible. These two capacitors carry the signal split by the phase inverter. Matching these two caps will pay off in a fantastic tone. Also, replacing the “Ground Grid” capacitor with a high-quality and higher voltage rated Orange Drop capacitor gives you a much better signal. I replaced the old 200V capacitor with a new 600V Orange Drop. Finally, matching the two 1M resistors in the circuit is important and a well-received modification.

I like to make biasing easy for me. I always add 1ohm (1w) resistors from pin 8 to ground on the power tubes. I can then measure bias amperage across this resistor by reading the voltage drop across it. It’s a simple mod and makes bias adjustments easy for power tubes.

Mod 8 – High Boost

I’m a big Silver Mica fan. I think the Silver Mic capacitors sound better than the old ceramic disc caps. I like to replace the high boost switch capacitor with the same value but using a Silver Mica capacitor. By doing so, you can really get that high-end sparkle especially with single coil Strat pickups.

Below is the work I completed:

•Replaced both high-boost switch capacitors (120pF Silver Mica)

Final Mods, Adjustments & Cleanup

Pot Cleaning

As always, I clean all the pots with good quality contact cleaner. This works 99% of the time. If a pot still has noise to it, I’ll replace it completely.

Soundboard Repair and Grill Cloth Replacement

The soundboard and grill clothe were a mess on this amp. The soundboard was barely attached to the frame and the grill cloth was worn and torn. This amp also included an aluminum trim around the grill cloth edge that was badly damaged. Fender added this aluminum trim on this model for about a year, but it was not well received and discontinued.

As you can see, I completely disassembled the soundboard. The screw holes were damaged and needed rework. I came up with a clever new way to mount the soundboard to the frame using 2" metal straps and T-nuts. This worked beautifully! I also installed new grill cloth. Although it doesn’t look “vintage” anymore, I think it turned out great.

Tube Replacement

A few tubes were bad and broken and needed replacing. Buying new tubes can be a gamble. You really never know what you are going to get. I try to stick with reportable brands. I’ve read so much on tubes that I am now more confused than ever. I've learned to take everyone else's opinions with a grain of salt. Experiment for yourself and judge which tubes are worth keeping, and which are worth throwing away. I personally stick with TAD, Electo-Harmonix and JJs. NOS Telefunken’s are very nice as well but can be expensive.

Below is what I replaced on this amp:

•Replaced power tubes with new TAD 6550s

•Replaced phase inverter (V6) tube with new EH 12AT7

•Replaced Reverb Tube (V3) with new EH 12AT7

•Replaced rectifier tube (V9) with new JJ GZ34S

Power Tubes – 6L6 Vs. 5881

The 6L6GC is the latest version of the 6L6 tube. This version has a 30-watt plate dissipation, 500V plate rating and 450V 5watt screen rating.

The 5881 (same as 6L6WGB) have a maximum plate rating of 360 volts at 23 watts and a maximum screen rating of 270 volts at 3 watts.

To my ear, I can’t tell the difference except that the 6L6s may break up a little sooner. Other than that, they sound the same to me.

Biasing

6L6's will generally bias properly somewhere between 48 to 55 volts in these amps. Biasing by current is better, if you're looking for clean sounds try between 30ma to 35ma per tube. Of course all the cautions apply. According to my bias setting tube chart in the Torres Engineering book, I need to set bias at 27-30ma per tube because my plate voltage 450V. I set bias at 28ma as a compromise. I am looking for the super clean Fender sound anyway.

Remaining Issues

Reverb Circuit Crackly Noise

There still seems to be some “crackly” noise when the reverb volume control is turned up. This occurs even if the reverb is switched off. I thought the tube was bad so I replaced it. There was some improvement but there still is a little bit of noise.

Tremolo Ticking Noise

There is a small ticking noise in the Tremolo circuit. It’s only apparent when the tremolo is turned on.

Add Mid Control

This is a nice modification that is easy to implement. I’ll perhaps try this at another time. I usually end up placing the pot in the Ext Speaker jack to avoid drilling new holes anywhere.

Conclusion

The Fender Pro Reverb Amp is underrated and under-appreciated by the guitar community. I think they immediately write it off because all production models were post CBS. In truth, it’s an awesome amp in the Fender evolution. 40 watts with two 12s is a great combination. The renovation and modifications really made this amp sing. It now has that classic “Fender Clean” punch that we all love. It’s also an amp that is very pedal friendly. I drive with my Zendrive or the Xotic BB Preamp and it really sings. The reverb is lush and it is plenty loud for any small club.